1. Field of the Invention
The present invention relates to a method and unit for setting a wavelength to a tunable laser that is used in a transponder unit provided in an optical transmission unit.
2. Description of the Related Art
In optical transmission techniques, a wavelength-division multiplexing (WDM) technique to transmit light signals of different wavelengths has lately been put to practical use and is increasingly evolving. For instance, as shown in FIG. 22, a WDM transmission unit includes a WDM section 100 and one or more transponder units 104. The WDM section 100 consists of a demultiplexing section (DMUX section) 101, a switching fabric (SW fabric) 102, and a multiplexing section (MUX section) 103. The transponder units 104 are connected with other low-speed transmission units and routers.
In the WDM section 100, the DMUX section 101 receives a wavelength-division multiplexed (WDM) light signal through a basic trunk and separates the WDM light signal into light signals of different wavelengths. The switching fabric 102 changes the destination of a light signal input for each wavelength, in the unit of a wavelength. For example, by provisioning, some of the light signals of different wavelengths from the DMUX section 101 can be dropped to the transponder units 104 or directed to the MUX section 103, and signals from the transponder units 104 can be added to the MUX section 103. The MUX section 103 combines the light signals of different wavelengths output from the switching fabric 102, into one WDM light signal. The WDM light signal is output onto a basic trunk.
On the other hand, the transponder unit 104 receives a light signal of one wavelength before multiplexing or after demultiplexing, and converts or monitors it for users. The transponder unit 104 has, for example, the function of performing the alarm/performance monitoring, line switching, and digital wrapping of a signal dropped from the WDM section 100 (DMUX section 101) or output from a downstream low-speed transmission unit, and the function of converting the wavelength of a light signal dropped from the WDM section 100, to a wavelength (e.g., 1.3 μm) for a downstream low-speed transmission unit, or converting the wavelength of a signal from the downstream, to a wavelength (e.g., 1.5 μm) to be added to a WDM light signal on a basic trunk.
For that reason, the transponder unit 104 is typically equipped with a tunable electro/optical (E/O) converter capable of selectively outputting light signals of different wavelength channels. With provisioning to a WDM transmission unit, a wavelength channel that an object transponder unit 104 uses is determined and wavelength data corresponding to that wavelength channel is set to the tunable E/O converter. The tunable E/O converter supplies a voltage, which corresponds to the set wavelength channel, to a built-in laser module after a predetermined time and sends out a light signal of a wavelength coincident with the set wavelength data.
Note that a conventional technique on laser modules is disclosed, for example, in Japanese Laid-Open Patent Publication No. 2001-196690. The object of the technique is to provide a laser system that is capable of stabilizing output wavelengths and making replacement of laser chips economic and easy. This technique makes an interchange of only laser chips easier by housing a laser chip (laser/memory module) and a memory device in different packages. This technique also makes the updating of operation of a new laser chip by a control system easy and quick, by storing the operating parameters (e.g., a laser bias current, a look-up table, etc.) required for the laser chip in the memory device and giving the required calibration value and operating data to the control system.
That is, when replacing an old laser/memory module, data (data about the initial and operating states of a new laser chip) stored in the memory device of a new laser/memory module is extracted and supplied to the control system. In this way, an old laser chip can be replaced with a new laser chip without performing the retest and recalibration of the laser system.
A conventional technique on laser control is disclosed in Japanese Laid-Open Patent Publication No. 2002-324933 by way of example. This technique provides a method of setting the peak value of the light quantity of a laser beam in which the wavelength is converted by a resonator (resonant cavity) formed in a semiconductor laser used as an excitation light source. In a temperature range where the output light quantity of the resonator can peak, temperature is gradually changed and a peak value is detected from the output light quantity data obtained at respective temperatures. A temperature corresponding to the peak value is set as a reference temperature at which the resonator is controlled. In this way, the output of the resonator can be controlled at the temperature where the light quantity peaks. In addition, a current value to the semiconductor laser does not need to be increased in order to compensate for an insufficient light quantity when the resonator is operating at temperatures other than the peak of the light quantity, so it becomes possible to save energy.
However, after long-time use of a tunable E/O converter, when wavelength data is reset to the tunable E/O converter because of insertion or removal of an object transponder unit, a power failure in a WDM transmission unit, resetting by provisioning, etc., the wavelength data at the time of initial setting is set to the tunable E/O converter. For that reason, if the tunable E/O converter is used for many hours, the corresponding relationship between the wavelength data and an actual output wavelength signal will be impaired. Because of this, if the wavelength data at the time of the previous setting is set, there are cases where an expected wavelength signal cannot be sent out.
That is, tunable E/O converters are typically equipped with an automatic wavelength correcting function, and if wavelength data for outputting a target wavelength λn is set to a data setting register provided in the tunable E/O converter, an internal laser diode (LD) emits light. The automatic wavelength correcting function monitors the output wavelength of the LD and checks whether the output wavelength is the target wavelength λn. If it is not the target wavelength λn, the wavelength data is updated so the output wavelength is the target wavelength λn. Whether the output wavelength of the LD is the target wavelength λn is determined by employing a wavelength filter that transmits only light of the target wavelength λn, and measuring the output intensity. In the case of four settable wavelengths, a wavelength filter for transmitting these four wavelengths is employed.
For example, as listed in Table 1, in a tunable E/O converter settable to λ1 to λ4, when the required target wavelength is λ2, initial wavelength data 0x2F8 (equivalent voltage 1.481 V) is set to the tunable E/O converter as wavelength data.
TABLE 1Initial data for λ1 to λ4 (fixed values)EquivalentChannel No.Initial data (HEX)voltage (V)λ1(1531.90 nm)0x4002.000λ2(1532.68 nm)0x2F81.481λ3(1533.47 nm)0x1A60.823λ4(1534.25 nm)0x0000.000
If the initial wavelength data is set, the LD starts emitting light at a wavelength of λ2, and the wavelength data is updated as needed by the automatic wavelength correcting function. For instance, consider the case where the equivalent voltages corresponding to λ2 and λ3 have become higher than the equivalent voltage of the initial wavelength data by about 0.6 V after long-time use. In this case, the equivalent voltage at the light emission of wavelength λ2 is about 2.0 V, and the equivalent voltage at the light emission of wavelength λ3 is about 1.4 V.
If the initial wavelength data 0x2F8 (equivalent voltage 1.481 V) corresponding to λ2 is reset to the tunable E/O converter because of insertion or removal of an object transponder unit, a power failure in a WDM transmission unit, or resetting by provisioning, the LD will emit light in the vicinity of λ3.
And since the automatic wavelength correcting function measures the output intensity of a wavelength filter that transmits λ1, λ2, λ3, and λ4, λ3 is recognized as the wavelength of a control object. As a result, light is emitted at λ3, not λ2. More specifically, the automatic wavelength correcting function receives the initial wavelength data of λ2, and fluctuates wavelength data in the vicinity of λ2 so that the output intensity of the wavelength filter is the maximum. Therefore, the automatic wavelength correcting function controls wavelength data so the output intensity at λ3 is the maximum.
Thus, the tunable E/O converter automatically updates wavelength data by the automatic wavelength correcting function, but if light is emitted at any one of the settable wavelengths, the light is transmitted through the above-described wavelength filter and the light emission at that wavelength is maintained. The automatic wavelength correcting function does not check whether the output wavelength is a target wavelength, so when the initial wavelength data is set, there is a possibility that depending on a difference between an actual output wavelength and the initial wavelength data, light will be emitted at a different wavelength. If a module capable of measuring an output wavelength is mounted in the tunable E/O converter, it becomes possible to recognize the output wavelength accurately. However, it is fairly difficult to mount the above-described module in the tunable E/O converter from the standpoint of size and cost.
For that reason, when the corresponding relationship between an actual output wavelength and initial wavelength data is impaired because of age degradation, etc., there is a possibility that the tunable E/O converter will recognize a different wavelength as a target wavelength and continue to output a light signal at an erroneous wavelength. As a result, in the worst case, a performance monitor error, signal disconnection, a unit failure, etc., will occur because of a shift in wavelength.
In the technique disclosed in the aforementioned publication No. 2001-196690, data about the initial and operating states of a new laser chip (e.g., a laser bias current, a look-up table, etc.) is stored in a memory device and is supplied to a control system, but after data is supplied, that data is fixedly used in order to operate a laser chip. For that reason, there is a possibility that a shift in wavelength due to age degradation will occur.
On the other hand, since the technique disclosed in the aforementioned publication No. 2002-324933 relates to a method of controlling temperature of an excitation light source (semiconductor laser), a shift in wavelength due to age degradation cannot be avoided.